Giant (Mar 2023)
A highly ordered 8/1 helical pyramidal column self-organized from the crown conformation of achiral hexa(butyloxy)triphenylene
Abstract
Liquid crystals of disc-like molecules are known for 45 years. 2,3,6,7,10,11-Hexakis(alkyloxy)triphenylene (HATn) containing n = 4 to 11 carbons in its achiral alkyl groups, represents one of the classic families of disc-like molecules. X-ray analysis suggested that disc-like molecules stack on top of each other at irregular spacing in columns distributed in the melt of their alkyl groups forming discotic or columnar or discotic nematic liquid crystals. Interest in discotic liquid crystals comes mostly from their conducting and photoconducting properties. While writing a recent perspective on the 45th anniversary of discotic liquid crystals we realized that the molecular structures of the 3D crystal and 2D liquid crystal phases of discotic molecules were never determined at the molecular level, in spite of the great interest in their physical properties. Since structure determines function, we decided to perform a detailed investigation of the molecular structure of HATn self-organizations by reconstructing their oriented fiber X-ray diffractograms both in the crystal and liquid crystal states. This communication reports the synthesis, analysis by differential scanning calorimetry of HATn containing n = 4 to 12 carbons in its achiral alkyl groups and the molecular structure of HAT4 in its crystal and liquid crystal states. Unexpectedly, a highly ordered 8/1 helical pyramidal crystalline column assembled from a crown-like conformation of the HAT4 and a non-helical column of the discotic liquid crystal state, the last similar to the original suggested supramolecular structure, were resolved for the first time at the molecular level. These preliminary results raise a lot of fundamental issues in the area of discotic and columnar self-organizations and of related helical columnar self-organizations generated with different building blocks and belonging to different disciplines. A comprehensive mechanistic elucidation of the molecular details of helical and non-helical self-organizations in synthetic complex systems may become as influential in the field of soft condensed matter as that of helical and non-helical self-organizations in the field of molecular biology.
